The invention relates to a method and an apparatus for film heating and to an associated measuring device for measuring the film temperature. The invention thus refers to thermoplastic films which are preferably produced, for example, on the basis of polyester, polypropylene or polyamide, the preferred materials, namely polyester, polypropylene or, for example, polyamide, not having to be in pure form, but the films also being capable of being produced, using modifications of these materials and/or, moreover, using further admixtures and/or additives.
Devices for the heating and thermal control of films, particularly also in the case of simultaneous stretching processes, are known.
Thus, for example, U.S. Pat. No. 5,429,785 describes a simultaneous stretching plant having a plurality of preheating and intermediate heating devices. These heating devices consist of hot-air heating devices or of infrared radiators. However, a combination of both may also be used for the heating and thermal control of the film. In the method, known from the abovementioned U.S. patent specification, for the production of ultrathin films with a final thickness of less than 2.5 xcexcm, the abovementioned heating devices can be provided at a plurality of locations in the stretching operation: they are in each case arranged transversely to the drawoff direction of the film web so as to cover the entire film width.
U.S. Pat. No. 5,071,601 discloses a method of producing a thermoplastic film. Here, the film is guided via a plurality of rollers slightly tapering conically, in order, via these, ultimately to produce a curved final film which is used, for example, as an intermediate film for multilayered glazing for motor vehicles. The heating device required for the process of producing the plastic film consists of a plurality of heating zones of an infrared heating device which are arranged, offset relative to one another, transversely to the drawoff path of the plastic film, the heating zones generating increasingly higher temperatures from one edge region of the film to the opposite edge region. This is necessary in order to achieve the desired curved film web profile mentioned. In this case, however, due to the different material stretching, the thickness of the film web has different values, although this is of only minor importance in the specific use and specific service of the plastic film web for the production of motor vehicle glazing.
It was then shown, however, that, in the simultaneous stretching of plastic films, although relatively uniform stretching is possible in the middle region of the film web, problems still arise, above all, in the film edge region. This is because, on the one hand, the film edge region always has a greater material thickness than the remaining middle film web portion. However, the stretching conditions are also impaired due to the fact that poorer heat transmission can be detected at the edge of the film web, this also being caused, inter alia, by the tenterhooks engaging on the film edge.
The object of the present invention is, therefore, to provide a method, as well as an apparatus for carrying out the method, and a suitable film temperature measuring device, which makes it possible to produce plastic film webs of improved quality in a stretching process.
Applicants have found that, in the stretching of films in longitudinal, transverse or simultaneous stretching processes, edges which are thicker, as compared with the middle of the film, must be thermally controlled separately before and during the stretching process, since, because of the greater thickness and the poorer heat transmission at the edge, the desired temperature cannot be achieved solely by blowing with standard nozzles. Another reason for the necessary different thermal control of the film edge and of the middle region of the film web is that the stretching behavior is different in each case. Thus, specifically, the film edge gripped by the tenterhooks is stretched essentially longitudinally, whereas the remaining film web material is stretched biaxially.
Furthermore, particularly in the case of simultaneous stretching, the film edge performs the essential function of the introduction and distribution of force. Since the rigidity of the edge can be influenced within wide ranges by means of the temperature, a defined setting and control of the edge strip temperature assumes appreciable importance.
In the previous stretching plants in general and the simultaneous stretching plants in particular, the film edge has not been thermally controlled separately. Moreover, hitherto, there have also been no solutions for subjecting this edge to special heating in a controlled manner. At the same time, the partitioning off of the film edge by the transport system, that is, above all, the tenterhooks engaging on the film edge at discrete intervals, presents a particular problem, since directional heating of the film edge is thereby impaired even further.
Admittedly, WO 94/047 WO has disclosed an apparatus using injector nozzles to blow textile cloth webs transported on, spread out, said apparatus comprising a blowing or nozzle box facing the top side and the underside of the cloth web. This textile drier is arranged transversely to the cross web. If special edge drying is desired, nozzles may also be directed only onto the edge regions of the cloth web or be subjected only there to treatment gas. To that extent, however, this is a nongeneric prior art, since the subject of the present application relates to an apparatus for the thermal control of plastic film webs during simultaneous stretching processes, and, in this case, specific temperature distributions must be achieved in the plastic material cross section.
Furthermore, DE 25 42 507 A1 has disclosed an apparatus for the zonal regulation of the thickness of a stretchable thermoplastic film web. In this known apparatus, separate film edge heating is not provided, but it is merely proposed to arrange above and, if appropriate, also below the film web parallelepipedic air wells which each have an inlet and an outlet for the hot air capable of being supplied, the throughflow quantity of the hot air supplied being variable in the individual wells by means of blind-like individually moveable slats. This known apparatus does not allow any actual separate film edge heating, however, since hot air having one temperature level can be made available only uniformly for the thermal treatment of the entire plastic film web. The temperature cannot be regulated zonally, only the hot air quantity. Moreover, heating is to take place solely by means of hot air, without the recognition that optimal film edge heating, and therefore temperature regulation for the film edge, can be achieved precisely by combined heat treatment by means of hot air and infrared irradiation.
The present invention affords numerous advantages. By virtue of the inventive edge strip heating for simultaneous stretching plants, the quality of the plastic film webs to be produced is markedly improved, as compared with conventional films produced. The material edges of a plastic film web, which are thicker as a consequence of production, can now be thermally controlled and heated directionally, in such a way that, in a simultaneous stretching process, this edge region of a plastic film web can also be optimally stretched. In order to achieve, overall, an optimal stretching condition for the film in terms of the running stability of the latter, on the one hand, and of the introduction of force from the film edge to the film web to be stretched, on the other hand, it is necessary for the film edge and the rest of the film material to be heated to the same temperature, despite the different thickness. By means of the inventively combined separate thermal control operation for the film edge by means of hot air and infrared radiation, almost any desired temperature profile can be set, and implemented, over the width of the film edge.
The use of infrared radiation for the heating and thermal control of the film edge is particularly suitable. This is because the film material can absorb the energy or output of the infrared irradiation over a substantially shorter distance, and therefore within a substantially shorter time, than hot air on account of the higher heat transmission. Particularly because short-wave rays (1.1. xcexcm) are used, the radiation penetrates more deeply into the film edge material. At the same time, a high output can be generated in a small space and be introduced into the film. Preferably concentrated radiation results not only in a high output, but also in heating over an exact area. In this case, however, the surface of the film could be damaged by an excessive introduction of heat, at the same time the underside of the film possibly still being below the temperature ranges to be achieved per se. The solution therefore lies in the simultaneous concentrated action of air at high velocity on the film edge. The hot air is set to a specific desired temperature, in such a way that the process is controlled via this so that rapid heating, which is uniform over the film thickness, is achieved (equalization of the heat introduced as a result of the infrared radiation). Particularly for special edge geometries with a thickness profile decreasing toward the middle of the film, the additional air is helpful and is important in order to prevent the thinner regions of the edge profile from being overheated by the infrared radiator and destroyed.
The factor essential for the introduction of energy is due to the infrared radiator. At the same time, the fact that the thicker film edge has a higher absorption behavior, as compared with the thinner film material portion adjoining it, has a positive effect. The absorption behavior is therefore dependent on the film thickness. This can be utilized particularly effectively in so-called bright radiators (radiator temperature above 2000xc2x0 C.). This means, for the heating operation, that the thinner film material transmits more radiation than the comparatively thicker film edge, so that the thinner film material portions adjacent to the edge therefore cannot be overheated, which would lead to tears during stretching.
However, a slightly higher temperature of the film cannot be avoided when the adjustable lateral blowing nozzles, preferably provided specially for film edge heating, are operated at the so-called process temperature. Thus, whereas, for example, the plastic film (with the exception of its edge regions) can be subjected overall to hot air in the corresponding heating zones at a temperature of, for example, 93xc2x0 C. (=process temperature), a process temperature set slightly lower for the hot air is preferably used in the edge region. However, due to the combination with the infrared radiator, a reduction in the air temperature for the lateral blowing nozzles to, for example, 90xc2x0 C. then makes it possible for the temperature between the edge and the remaining film material to be set very accurately to an almost constant desired temperature level.
The inventive advantages of almost entirely constant thermal control of a film over the entire width are afforded in a combination of infrared radiation and convection (blowing with hot air), even when this combined thermal control operation is carried out from only one side (for example, the top side of the film). Even here, the temperature profile is actually set equally constantly and uniformly over the entire film thickness. In this case, maximum temperature deviations of, for example, 2xc2x0 K. or 1xc2x0 K. over the entire film edge thickness are possible.
A measuring device according to the invention for the appropriate setting of the desired temperature profile even in the film edge region is distinguished by the use of a pyrometer, the setting time of which is such that the interchange between tenterhooks moving past and film does not lead to a signal fluctuation. If, furthermore, measurement, determination or presetting of the temperature of the tenterhook is taken into account, the edge strip temperature can ultimately be ascertained comparatively accurately from these data and may then serve again as an initial control variable for activating the heating device.